1. Field of the Invention
This invention resides in the field of laboratory apparatus for performing procedures that require simultaneous temperature control in a multitude of small samples arranged in a geometric array. This invention is of particular interest in systems utilizing unitary contoured multiple sample supports, commonly known as “sample blocks,” in conjunction with thermoelectric modules for modulation and control of the temperature of the entire block or a section of the block.
2. Description of the Prior Art
The polymerase chain reaction (PCR) is one of many examples of chemical processes that require precise temperature control with rapid temperature changes between different stages of the procedure. PCR amplifies DNA, i.e., it produces multiple copies of a DNA sequence from a single copy. PCR is typically performed on a multitude of samples simultaneously in parallel manner, using instruments that provide reagent transfer, temperature control, and optical detection in a multitude of reaction vessels such as wells, tubes, or capillaries. Each sample in the process undergoes a sequence of process stages that are temperature-sensitive, with different stages performed at different temperatures and maintained for designated periods of time, and the sequence is repeated in cycles. Typically, a sample is first heated to about 95° C. to “melt” (separate) double strands, then cooled to about 55° C. to anneal (hybridize) primers to the separated strands, and then reheated to about 72° C. in a reaction mixture that contains nucleotide bases and DNA polymerase to achieve primer extension. This sequence is repeated to achieve multiples of the product DNA, and the time consumed by each cycle can vary from a fraction of a minute to two minutes, depending on the equipment, the scale of the reaction, and the degree of automation.
Nucleic acid sequencing is another example of a chemical process that involves temperature changes and a high degree of control, different temperatures being required for such steps as the denaturing and renaturing of the nucleic acid as well as enzyme-based reactions.
The successful performance of PCR, nucleic acid sequencing, and any other processes that involve a succession of stages at different temperatures requires accurate temperature control and fast temperature changes. As noted above, many of these processes involve the simultaneous processing of large numbers of samples, each having a relatively small volume, often on the microliter scale. In some cases, the procedure requires that certain samples be maintained at one temperature while others are maintained at another temperature, thus requiring the maintenance of different regions of the block at different temperatures and in some cases a temperature gradient. In both PCR and nucleic acid sequencing, the automated laboratory equipment that controls the temperature is known as a thermal cycler, and as noted above, many automated systems utilize a sample block with a multitude of wells arranged in the block in a geometrical array. The wells are either used as individual reaction vessels for each of the samples by placing the samples directly in the wells, or as a support for a disposable plastic plate which itself contains an array of wells conforming in shape to the wells of the block. When a disposable plate is used, the plate is placed directly over the block with the contours of the plate and the block in full contact. The wells in the plate then serve as the reaction vessels while the underlying block provides rigid support to the plate and close temperature control due to the intimate surface contact.
The temperature of the sample block in many of these systems, and hence the temperatures of individual samples, are usually modified by the use of thermoelectric modules, although electrical heating, air cooling, liquid cooling, and refrigeration can also be used. Thermoelectric modules are semiconductor-based electronic components that function as small heat pumps through use of the Peltier effect, causing heat to flow in a direction determined by the direction in which electric current is passed through the component. Thermoelectric modules are particularly useful due to their ability to provide localized temperature control with fast response, and to the fact that they are driven electronically which provides a high degree of control. The modules are typically arranged edge-to-edge with their heat transfer surfaces in full contact with the flat undersurface of the sample block.
Thermoelectric modules and any components that serve as heat exchange units function most effectively when pressed tightly against the sample block. For optimal thermal response, a sample block must be stiff and made of a material that has a high heat transfer coefficient and a low thermal mass. Stiffiess also benefits the reactions themselves by keeping the wells in planar alignment and preventing the block from bowing or otherwise becoming distorted in response to the applied mechanical pressure. The rate at which the samples in the wells are heated or cooled will vary with the mass of the block. The lower the mass of the block, the faster the temperature changes are transmitted to the samples. Thus, while metals such as aluminum offer the requisite stiffness, particularly near the bottom surface of the block, their mass retards the heat transfer to the samples. This is true whether the samples reside in the wells of the block or in a disposable plate in contact with the block. These and other concerns are addressed by the present invention.